Physics · Ch 11 — Waves
Applications of Reflection of Sound Waves
Applications of Reflection of Sound Waves
Four everyday and technological applications rest directly on the reflection of sound waves. The stethoscope works by exploiting multiple successive reflections of sound: it consists of a small disc-shaped chest piece containing a diaphragm resonator highly sensitive to sound (which amplifies the faint sounds it detects, such as heartbeats or breathing), connected via a length of rubber tube to metal ear pieces; the sound picked up at the chest piece is guided along the full length of the connecting tube, and around any bends in it, purely by a chain of repeated internal reflections off the tube's inner walls, ultimately reaching the listener's ears clearly. An echo is the perceived repetition of a sound heard after it reflects off a distant surface -- a wall, a mountainside, or any other obstructing surface -- and returns to the listener; since the human ear can only distinguish two separate sounds as genuinely distinct if they arrive at least of a second apart, a reflecting wall must lie at least about 17.2 m away (computed from , using air's speed of 344 m/s at 20 degC) for a returning echo to be perceived as separate from the original sound rather than blending into it. SONAR (SOund NAvigation And Ranging) uses the reflection of sound pulses travelling through water to locate the position or motion of underwater objects by timing the round trip of a reflected pulse; dolphins and bats use exactly the same underlying sonar/echolocation principle to navigate in darkness or murky water. Reverberation is the phenomenon, inside a closed room, of sound being repeatedly reflected from the walls such that it remains audible for some time even after the original source has stopped producing it; the length of time this residual sound persists is called the reverberation time, and because …
What this figure shows. A stethoscope is drawn showing its three main parts: a disc-shaped chest piece (containing a sensitive diaphragm resonator) placed against the body, a length of flexible rubber tube connecting it to metal ear pieces worn by the listener, and inside the tube a series of small zig-zagging arrows are drawn to represent sound repeatedly bouncing off (reflecting from) the tube's inner walls as it travels along its length from the chest piece to the ear pieces. The figure explains visually how the faint sound of a heartbeat or breathing, picked up and amplified by the sensitive chest-piece diaphragm, is able to reach the listener's ears clearly despite the tube's many bends: it is guided along the tube's full length purely by a chain of repeated internal reflections off the tube walls, exactly analogous to how light …
Worked out. A man standing some distance from a cliff claps his hands and hears the echo return after 4 seconds; taking the speed of sound as 343 m/s, the task is to find the distance between the man and the cliff. Since the sound must travel from the man to the cliff and then reflect all the way back before being heard as an echo, the 4-second interval is the time for the ROUND TRIP, so the one-way travel time is half of that, . The distance then follows directly from . The example is a direct, worked illustration of the general echo-timing principle: because the measured time interval always corresponds to sound completing a full there-and-back journey, it must always be halved before being multiplied by the …